A fault runs through it: Montclair

The Hayward fault runs through the heart of Montclair, in the Oakland hills behind Piedmont. Montclair Park’s duck pond was constructed where the fault left a natural sag in the ground.

Only in seismologists’ equations, and perhaps deep down in the crust, are faults smooth, flat planes. In the world, on the surface, faults are as ragged and variable as any other geological feature. The Hayward fault is more of a zone, from a few to a hundred meters in width, with several fractures running through it. Where two strands overlap, a block of ground between them may slump in tension or rise in compression, depending on how the strands are oriented. At Montclair Park, two strands are mapped on either side of the sag basin. Lake Temescal is another example of a sag basin repurposed as a water feature. So, apparently, is Lake Aliso, the pond on the grounds of Mills College.

The first “great San Francisco earthquake” occurred 21 October 1868 on the Hayward fault. The epicenter appears to have been in southern San Leandro, and surface rupture extended from there all the way down to Fremont. In Montclair, the other direction along the fault, there was plenty of shaking of course, but no rupture of the ground from what we can tell. A trenching study, conducted along the third-base line of the little ballfield in Montclair Park, found no sign of recent movement along the fault there. But slow, silent motion does affect the fault in Montclair. The old fire station on Moraga Avenue has been rendered useless by aseismic creep, and some of the houses along the fault appear to show foundation disruption. But generally creep is invisible unless there is some structure that it affects, like the south curb of Medau Place, below. South of here, the fault crosses Route 13, reaching the other side at the head of Dimond Canyon.

I can’t see anything in the lower pic that I wouldn’t attribute to root action or even just old sidewalks in non-seismically active areas, am I missing something in the flat picture that is easier to see in 3D?

Well, there’s a right-lateral offset of about 8 inches exactly where the fault is mapped, and similar offsets are seen in adjoining streets. Not as noticeable as the offsets in Hayward, but creep in Oakland is only a fraction of what it is in Hayward.